Thermogravimetry and Gas Analysis, Part 1: Basic Principles and Overview

A thermobalance coupled to a suitable Evolved Gas Analysis (EGA) system allows qualitative information to be obtained about the gaseous reaction or decomposition products formed in a TGA experiment in addition to purely quantitative information about mass changes. This new series of articles discusses the various measurement techniques that METTLER TOLEDO offers for such analyses.

Introduction

In thermogravimetric analysis, the mass of a sample is continuously measured as a function of temperature. Changes in mass of just a few micrograms can be detected with high accuracy. Modern thermobalances such as the METTLER TOLEDO TGA/DSC 1 can also simultaneously measure the heat flow to and from the sample. This provides information about thermal changes that occur during the mass change and also about effects that are not associated with a change in mass such as melting, crystallization, or solid-solid transitions.

Questions regarding the identity of the gaseous products evolved during the mass change however remain unanswered. This information can be obtained by coupling the TGA to a suitable system for gas analysis. In this five-part series of articles, we will discuss the various techniques that METTLER TOLEDO offers for such analyses. The first part presents an overview of the different techniques and discusses their application possibilities.

In the four articles that follow, we will show how TGA-MS (thermogravimetric analysis coupled with mass spectrometry), TGA-FTIR (thermogravimetric analysis coupled with Fourier transform infrared spectroscopy), TGA-GC/ MS (thermogravimetric analysis coupled with gas chromatography and mass spectrometry), and TGA-Micro GC(/MS) (thermogravimetric analysis coupled with micro gas chromatography and (optionally) mass spectrometry can be applied in practice.

The Various Techniques

gas analysis techniques discussed here all have something in common, namely that the gases and volatile products evolved during the heating process in the TGA have to be transferred to the gas analysis system. This is accomplished using a transfer line. This is typically maintained at a temperature of 200 °C to prevent gaseous products from condensing.

In TGA-GC/MS, the decomposition gases are however normally not measured online, in contrast to TGA-MS, TGA-FTIR and TGA-Micro GC(/MS). Instead, gas samples are taken at particular furnace temperatures during the TGA measurement and stored in a special interface. The gas samples are analyzed afterward by GC/MS after the TGA measurement has finished (for further details, see page 5).

Conclusions and recommendations

TGA-MS, TGA-FTIR, TGA-GC/MS and TGA-Micro GC(/MS) are powerful techniques that yield both quantitative (mass loss) and qualitative (identification) information about the gaseous products released during a TGA measurement. Table 1 presents a comparison of the four methods.

Not all techniques are equally suitable for dealing with specific questions. The overview in Figure 15 shows which technique is best for solving a particular application problem.

The GC/MS multi injection mode is a special operating mode of the GC in which the separation power of the GC is greatly reduced. The advantage of this operating mode is that GC/MS can be performed online like MS or FTIR.

We will give more detailed information about the individual analysis techniques in future TA Tips on this topic.

_{Thermogravimetry and Gas Analysis, Part 1: Basic Principles and Overview | Thermal Analysis Application No. UC 451 | Application published in METTLER TOLEDO Thermal Analysis UserCom 45}